The present invention relates to films for packaging of food or articles, particularly to wrap films for food. More specifically, the invention pertains to crosslinked laminated polyethylene wrap films excellent in adhesion, pulling-out ease from a dispenser box and heat resistance, emits little odor, can wrap food suitably and have good production efficiency.
Wrap films for food are used mainly at home as a convenient packaging material for food and most of them serve to protect food. Such wrap films are required to have adhesion between a container and the film or between the films high enough not to cause peeling by itself. They are also required to have heat resistance durable to the use in a microwave oven, elastic touch, resilience, good appearance when wrapped with the film, hygienic and low odor emission. Most of such wrap films are placed in a dispenser box as a film roll so that low power for pulling out from a dispenser box, low power for cutting and stable propagation property are required at the same time. In addition, efficient production is important for providing good quality and inexpensive wrap films.
The above-described adhesion is necessary for preventing a film, with which food or a container having food therein has been wrapped, from peeling by itself, thereby coming the food or container in contact with dust or another article (food). The pulling-out power, on the other hand, must be sufficiently low in order to provide users with handling ease. Lack of either one of them lowers the function as a wrap film so that it is necessary to satisfy these two performances simultaneously.
In order to maintain satisfactory appearance of a wrapped article, bubbling as described in Japanese Patent Laid-Open No. 286087/1999 must be suppressed. This bubbling is a phenomenon of air bubbles, which have been formed by boiling of water contained in the wrapped article during heating in a microwave oven, remaining on the surface of the wrap film even after heating. The bubbling deteriorates the appearance of the wrapped article. More specifically, water contained in the article wrapped by the film evaporates during heating, adheres to the film and becomes dew drops. These dew drops are boiled into air bubbles. The surface tension of these air bubbles is increased by a surfactant added to the film so that air bubbles remain even after heating.
To satisfy the above-described performances, various improvements of a polyethylene film have been made in a known manner. For example, proposed for imparting a wrap film with necessary adhesion are a process of adding an ethylene.vinyl acetate copolymer or tackifier to its surface layer resin, and a process of using a ultra-low density polyethylene or linear low density polyethylene which is viscous and contains a large amount of a low-molecular-weight component. Known are a process of adding a lubricant such as polyhydric alcohol fatty acid ester to lower the pulling-out power, a process of adding a high density polyethylene to impart the film with resilience and elasticity and a process of carrying out crosslinking treatment in order to impart the film with heat resistance. Moreover, a technique of a higher level has been developed to satisfy the balance between the above-described properties necessary for a wrap film by using these techniques in combination. For example, as a film other than a wrap film, Japanese Patent Publication No. 21930/1993 proposes an ethylene-based crosslinked film having layers different in the content of a polyhydric alcohol fatty acid ester, whereby layers constituting the film are different each other in the crosslinking degree. Here, the polyhydric alcohol fatty acid ester is used as a crosslinking controlling agent. As a wrap film, Japanese Patent Laid-open No. 25355/1998 discloses a wrap film made of a metallocene-based ethylene copolymer, Japanese Patent Laid-Open No. 286087/1999 discloses a crosslinked laminated film obtained by adding a glycerin trifatty acid ester to each of the surface layers containing a linear low density polyethylene and the intermediate layer containing a high density polyethylene. U.S. Pat. Nos. 5,019,315 and 5,248,547 disclose stretched wrap films each having a linear low density polyethylene as a surface layer and a high pressure process low density polyethylene as an intermediate layer; and preparation processes thereof.
The techniques however involve the below-described drawbacks. The film described in Japanese Patent Publication No. 21930/1993 is accompanied with the drawback that it contains a large amount of a polyhydric alcohol fatty acid ester having also a function as a surfactant so that air bubbles remain on the film surface when heated in a microwave oven. The laminated film according to Japanese Patent Laid-Open No. 255355/1998 contains a metallocene-based polymer so that it has high tear strength and is inferior in cutting property. The films disclosed in U.S. Pat. Nos. 5,019,315 and 5,248,547 have poor heat resistance, and owing to a high proportion of a high pressure process low density polyethylene in the whole film, has lowered elasticity (resilience and elasticity). Japanese Patent Laid-Open No. 286087/1999 discloses a wrap film added with a glycerin trifatty acid ester in order to improve adhesion, pulling-out property, cutting property and heat resistance. Upon crosslinking a coextruded parison and stretching the resulting parison by a double bubble method, contamination resulting from a glycerin trifatty acid ester is generated much on a pinch roller rightly downstream of the second bubble and is deposited on a film, thereby deteriorating the product (which will hereinafter be called film contamination). In order to prevent it, production is interrupted and contamination is removed frequently from a roller, which worsens production efficiency and leads to a cost increase.
As described above, a polyhydric alcohol fatty acid ester or glycerin trifatty acid ester added to prepare a film having a good quality causes unexpected problems such as deterioration in the appearance of a wrapped article or generation of film contamination.
Another problem exists in the balance of the performances of the film. When an increase of adhesion is intended, pulling-out power becomes higher. When lowering of the pulling-out power is intended, on the other hand, the adhesion lowers. An increase in the modulus of elasticity, which is an index of elasticity or resilience, inevitably worsens stretching property. Thus, it is very difficult to maintain the balance between these properties.
With regards to cutting property, optimization of the combination of a material or constitution of a film and a cutter has hardly been conducted.
An object of the invention is to overcome the above-described problems and to provide a crosslinked laminated polyethylene wrap film which is excellent in adhesion, pulling-out ease from a dispenser box, cutting ease, heat resistance, emission of odor from the film and touch feel, has less film contamination upon preparation of the film, does not leave air bubbles after heating in a microwave oven and is capable of wrapping food suitably.
Another object of the invention is to provide a dispenser box suitable for having the wrap film disposed therein.
Other objects and effects of the invention will become apparent from the following description.
The above-described objects of the invention have been achieved by providing a crosslinked laminated wrap film having, at least, two surface layers and one intermediate layer,
wherein the two surface layers each comprising 20 to 90% by weight of a linear low density polyethylene and 80 to 10% by weight of a high pressure process low density polyethylene,
wherein the intermediate layer has at least one layer comprising 50 to 95% by weight of a high density polyethylene and 50 to 5% by weight of a high pressure process low density polyethylene,
wherein the surface layers and the intermediate layer each has 0.050 to 0.800 part by weight of a glycerin monofatty acid ester based on 100 parts by weight in total of the two polyethylenes constituting the respective layer.
The invention differs from the conventional techniques in the following points: according to the conventional techniques, various ethylene (copolymers are used for wrap films and in some cases, a polyhydric alcohol fatty acid ester is added in a large amount, while in the crosslinked laminated wrap film according to the invention, the two surface layers each comprises 20 to 90% by weight of a linear low density polyethylene and 80 to 10% by weight of a high pressure process low density polyethylene; the intermediate layer comprises 50 to 95% by weight of a high density polyethylene and 50 to 5% by weight of a high pressure process low density polyethylene; and each of these three layers has 0.050 to 0.800 part by weight of a glycerin monofatty acid ester based on 100 parts by weight in total of the two polyethylenes constituting the respective layer.
The role of the constituent elements of the invention differing from the conventional techniques is to impart a crosslinked laminated polyethylene wrap film with excellent adhesion, pulling-out ease a dispenser box, cutting ease, good touch feel with elasticity, excellent heat resistance and low odor emission property. In addition, it features high resilience in a dispenser box, free from air bubbles, good appearance of an article wrapped with the film and little film contamination upon film formation.